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Autumnal Equinox 2015

Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially began at 8:21 UTC, which is 4:21 AM Eastern Daylight Time.

The astronomical seasons are a product of the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. During the autumn months, the Earth’s axis is tilted neither toward nor away from the sun. This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across our sky has been moving southward and daylight hours have been decreasing. Today, it crossed the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

With the sun sitting lower in the sky and daylight hours continuing to shorten, Autumn is a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA

The Earth’s axis is tilted neither toward nor away from the sun on the Equinox. Image Credit: NASA